1. Field of the Invention
The present invention relates to a display device using a light emitting element. In addition, the present invention relates to an electronic device including the display device in a display portion.
2. Description of the Related Art
In recent years, a technique of forming a transistor, such as a TFT (thin film transistor), over a substrate has been drastically developed, and development of an active matrix display device has been promoted.
In addition, a so-called self-luminous display device has been attracting attention, which has pixels each formed using a light emitting element such as a light emitting diode (LED). As a light emitting element used in such a self-luminous display device, there is an organic light emitting diode (also referred to as OLED), an organic EL element, an electroluminescence (EL) element, which have been attracting attention and started to be used for an organic EL display or the like. Since the light emitting element is a self-luminous type, it does not require a light source such as a backlight, unlike a liquid crystal display device. Accordingly, such a light emitting element is expected to realize more lightweight and thinner display devices. In recent years, development of a wide-screen EL display has been promoted, following a liquid crystal TV.
When putting an EL display into practical use, a short life of a light emitting element because of deterioration of an EL layer has been a problem. As factors affecting the length of the EL layer life, a structure of a device that drives the EL display, a characteristic of an organic EL material constituting the EL layer, a material of an electrode, conditions of the manufacturing steps, and the like can be given.
In addition to the factors given above, a driving method of the EL display has been attracting attention as one of the factors affecting the length of the EL layer life. In order to make an EL layer emit light, a method in which direct-current electricity is supplied to an anode and a cathode sandwiching an EL layer has been conventionally used. In other words, the EL display is driven with a direct current, and the direction of an EL driver voltage applied to the EL layer is always the same.
However, a driving method in which a forward driver voltage and a reverse driver voltage are applied to the light emitting element, and a current sufficient enough to insulate a short-circuited point can be supplied to the short-circuited point when a reverse driver voltage is applied to the light emitting element, so that the life of the light emitting element can be extended is proposed (see Patent Document 1: Japanese Published Patent Application No. 2005-202371).
Furthermore, there is an initial failure in which a pixel electrode and a counter electrode are short-circuited and a region where light is not emitted is formed in a pixel region. Short-circuiting occurs in the following cases: a foreign substance (dust) attaches before formation of a light emitting element; a minute projection is generated in an anode when the anode is formed, and a pinhole is generated in an electroluminescent layer; an electroluminescent layer is not formed uniformly and a pinhole is generated since a film thickness of the electroluminescent layer is thin; and the like. In a pixel where such an initial failure occurs, lighting and non-lighting in accordance with a signal are not performed, and almost all the current flows in the short-circuited point and a phenomenon that the element as a whole stops lighting occurs, or a phenomenon that a particular pixel lights or stops lighting occurs; therefore, display of an image is not performed well.
Other than the above-described initial failure, a progressive failure (also referred to as time degradation) that is caused by newly generated short-circuiting of an anode and a cathode over time sometimes occurs. The short-circuiting of the anode and the cathode that is newly generated over time occurs due to a minute projection that is generated when the anode is formed. In other words, a potential short-circuited point exists in a stacked body in which an electroluminescent layer is sandwiched between a pair of electrodes, and the short-circuited point comes out over time. Furthermore, other than short-circuiting of the anode and the cathode, the progressive failure is said to be generated when a minute space between the electroluminescent layer and the cathode expands over time, and a connection failure between the electroluminescent layer and the cathode is caused.
By applying a reverse driver voltage, the short-circuited point is carbonized or oxidized; thereby insulated, so that an initial failure can be prevented from developing further. A progressive failure can also be prevented from being generated or developing, by insulating the short-circuited point by carbonization or oxidation, or by suppressing the expansion of the space between the electroluminescent layer and the cathode.
In order to suppress development of a failure, a light emitting element needs to be driven with an alternating current. Driving a light emitting element with an alternating current means that voltages with different polarities are applied to the light emitting element alternately. In other words, a reverse voltage is applied to the light emitting element, in addition to a forward voltage which is required for light emission. Intensity and applying time are not necessarily the same between the forward voltage and the reverse voltage. Even the case where the amount of a reverse voltage to be applied is very small is referred to as an alternating current. In the present invention, a reverse voltage is applied to a light emitting element, and the light emitting element is AC-driven by applying a reverse bias current; thereby suppressing a failure of the light emitting element.
In order to insulate a short-circuited point, a large enough current to insulate the short-circuited point needs to be applied. Usually, the value of a large enough current to insulate a short-circuited point is desired to be much larger than the value of a current flowing in a forward direction to let a light emitting element emit light.
On the other hand, in an already established inexpensive manufacturing technique, a display device using amorphous silicon and a driving method have been issues. In the case where poly-silicon is used for a semiconductor film, for example, a process of crystallization is required. However, it is difficult to uniformly irradiate a large-area substrate with laser light; therefore, it is difficult to obtain uniform crystals over a large area. Accordingly, manufacture of a high-quality display device using amorphous silicon which enables enlargement of the area and does not require crystallization, and of which manufacturing process is simple, and a driving method thereof have been developed. However, in the case where amorphous silicon is used, the display device needs to be constituted by an N-channel transistor, since a P-channel transistor cannot realize sufficient operating characteristics and function.